A new work item, “LTE Carrier Aggregation Enhancements”, was agreed in RAN plenary meeting (RAN#52, May 31 to Jun. 3, 2011) for Rel-11 LTE. One of the main objectives of this work item is to study additional carrier types including non-backwards compatible elements for carrier aggregation, considering their deployment scenarios, benefits, drawbacks and standardization impacts. The additional carrier types were also briefly considered during Rel-10 LTE work (see reference [2] below). The following two additional carrier types were considered:                Extension Carrier        Carrier segment        
As shown in FIG. 1A, the carrier segment has a backward compatible carrier (stand alone carrier) and one or two carrier segment(s) located at back side only or front and back sides of the backward compatible carrier. The extension carrier has, as shown in FIG. 1B, a backward compatible carrier (primary component carrier) and a extension carrier (secondly component carrier) located thereafter.
However, these additional carrier types were not further considered for Rel-10 LTE mainly due to anticipated work load in 3GPP RAN work groups and available time left for Rel-10 LTE completion (see reference [3] below). Many company contributions in last RAN1 meeting (RAN1#66, Aug. 22 26, 2011) discussed the benefits of additional carrier types, and proposed to be standardized in Rel-11 LTE (see references [4]-[8] below). The key motivations for additional carrier types are:
Flexible and efficient spectrum usage so as to:                Utilize available spectrum for legacy UE operation, while introducing new bandwidths and features for new UE operation; and        Optimize time and frequency resources (i.e., resources elements) for data transmission, while reducing the over head for control and reference signal.        
Interference coordination and management so as to                Migrate from cell specific transmission in Rel-8/9 to UE specific transmission from Rel-10 onwards, which is more efficient form interference coordination point of view in heterogeneous network deployment scenario.        
Energy saving so as to:                Move from cell specific transmission to UE specific transmission to avoid transmission unnecessary control and reference signals; and        perform Demand based transmission.        
“Extension Carriers” and “Carrier Segments” would be characterized by reference [2] as:                No PBCH/Release-8 SIB/Paging        No PSS/SSS        No PDCCH/PHICH/PCFICH        No CRS        Rel-10 mobility is based on measurements in backwards compatible CC(s)        
An extension carrier must be a part of a component carrier set where at least one of the carriers in the set is a backwards compatible component carrier. A carrier segment is defined as contiguous bandwidth extension of a backwards compatible component carrier. Moreover, carrier segments would have single HARQ for the combined bandwidth with single PDCCH for resources allocations (see reference [2]).
In fact, one could consider “Extension Carrier” as a sub-set of “Carrier Segment” where bandwidth of the backward compatible part of the carrier is zero. Thus, “Extension Carrier” always needs to be attached with primary component carrier (PCC) for synchronization and mobility mechanisms. To simplify the discussion, we call additional carrier type with “Carrier segment” as “semi-backward compatible carrier (SBCC)”. Note that SBCC can operate either as standalone carrier or as one component carrier in the carrier aggregation scenario.